1. Field of the Invention
This invention relates to line tracking systems for automatically directing a vehicle along a predetermined path and more particularly to a line tracking system capable of tracking an invisible guide line.
2. Description of the Prior Art
A variety of automatic control devices for vehicles have previously been developed. The principal use for such automatic guidance systems has been in connection with industrial vehicles of the type that perform delivery or work functions in an industrial or office facility. Systems also have been developed for automobile guidance on highways but no such systems have been implemented on a commercial basis.
The basic types of guidance systems previously used on unmanned industrial vehicles have been permanent tracks, buried wire, reflected light, and radio controlled.
Track vehicles are vehicles that ride along a fixed rail or a track and are guided by the contour of the track. These vehicles are used primarily for automated storage-retrieval systems such as lift trucks that move on tracks between many-leveled storage racks and stock or retrieve material under computer control. This technique is unacceptable for floor maintenance machines (i.e., machines that scrub, sweep, or vacuum a floor) or any other application except for highly structured environments, such as stadiums, theaters, or stairwells. The presence of a fixed or permanent track on many floor surfaces is aesthetically as well as physically undesirable, and the installation of such a track is a substantial expense. Further, when a permanent track is installed, it is difficult and expensive to change the location of the track, thereby reducing the flexibility of the system.
Buried wire guidance systems have been applied successfully to various material handling systems, including hospital food trays, office mail carts, and automotive parts for use on assembly lines. In such a system, a wire is buried in the floor along the track the vehicle is to follow, and both ends of the wire are terminated at an excitation source. The wire is excited with an alternating current (typically 10 kHz) and its radiation pattern is then detected by two sensing coils on the vehicle. The differential output of these coils is utilized to drive a steering mechanism so that the vehicle is steered to follow the buried wire. This technique has serious limitations, one of the principal limitations being that it is generally not practical to employ a buried wire system in an existing structure. To apply a buried wire system to an existing floor structure, it would be necessary to cut a long groove in the floor structure along the path the vehicle is to follow and lay a new floor surface over the wire after it is buried in the groove. Even if the expense of burying a wire in an existing floor structure could be justified for a given application, the system would lack flexibility because to change the position of any given line, it would be necessary to cut a new groove in the floor and lay a new wire in this groove, while at the same time disconnecting the old wire.
Other disadvantages with a buried wire system are that the materials are relatively expensive, as well as being difficult to install and remove, and interference problems could be encountered when a buried wire system is used in a floor that is heavily reinforced with metal reinforcing rods.
Buried wire guidance systems are not appropriate for floor care machines for all of these reasons--they are too expensive, and the system contains little flexibility with regard to changing the placement of the guide line.
In reflected light sensing systems, a brightly visible line that contrasts markedly with the surface of the floor is laid along a predetermined path to be followed by the vehicle. The vehicle includes a light source of visible light that shines on the floor and photocell sensors that detect the level of light reflected from the floor (i.e., both the background and guide line illumination). The sensors are connected to a differential output amplifier much in the manner of the sensing coils in the buried wire system. The differential output is connected to a servo drive mechanism, which steers the vehicle along the guide line as the vehicle moves along the floor. One of the problems with the reflected light system is that the background or spurious illumination from the floor also is received by the photocells, and this illumination impairs the ability of the system to detect the contrast between the guide line and the background illumination. With surfaces having different contrast levels, such as a tile surface employing black and white checkered tiles, accurate tracking with a reflected light sensing system is unreliable.
Indeed, reflected light guidance systems have been known to cause a vehicle to follow a beam of sunlight that is cast across a floor surface from a window. Such unreliability is a critical defect in automatic machines because unreliable machines require constant supervision (thus eliminating labor cost savings), and an unmanned machine out of control on a spurious or non-existent line can be extremely hazardous.
One of the positive attributes of a reflected light system is that an existing line can be removed or moved to a new location easily, thus defining a new guide line for the vehicle to follow. However, the very impermanence and visibility of such a guide line also has drawback. A visible and removable guide line invites vandalism, and such guide lines are aesthetically unappealing. The aesthetic unacceptability alone is fatal to the use of a reflected light system in many applications, one of which is in the floor care industry. It would be completely unacceptable to almost any operation to have a complete matrix of highly visible lines crisscrossing on a floor surface just so that a floor scrubber or a floor sweeper could be operated automatically.
Another problem with a reflected light tracking system is that such systems are effectively limited to use on hard, flat surfaces, such as concrete or tile. Problems with obtaining the necessary reflectance levels and with the aesthetic undesirability of a solid, bright visible line on a carpet would prevent the application of such a system in any such environment.
Notwithstanding the fact that the visible line system and the other systems designed above have been in existence for many years (visible line systems having been in existence since at least 1937) heretofore no system has been developed which obviates the disadvantages of the three systems discussed above and provides a suitable automatic guidance system for use in connection with floor care machinery or other types of machinery, wherein a high degree of system reliability is essential, and system flexibility for rerouting the guide path and line invisibility are at least desirable. It is a principal object of the present invention to provide an automatic guidance system for a vehicle that is highly reliable, and is capable of following a guide line that is substantially invisible under normal ambient lighting conditions, and can be applied and reapplied easily to the surface of an existing floor.